This invention relates to methods and apparatus for countergravity casting of metals, particularly of reactive metals, such as certain alloys, containing elements which are reactive at casting temperatures with air to form oxides and/or nitrides, harmful to the cast product.
For good quality casting of such reactive metals, all casting operations, during which the metal is at a temperature at which it is reactive with air, should be conducted with air excluded, either by means of a vacuum or by air replacement with an inert gas. A method and apparatus in general use for countergravity casting with such air exclusion is disclosed in U.S. Pat. Nos. 3,863,706 and 3,900,064. According to these patents, an air-tight, evacuable compartment is mounted above an air-tight enclosure of a melting crucible for the metal, the compartment having access to the crucible through an air-tight conduit extending from the compartment through the top of the crucible enclosure, the conduit being provided with an intermediate closure slide valve. Thus, the crucible enclosure is not exposed to the ambient atmosphere. A vertically partable chamber within the compartment receives and is sealed around a pervious mold with the lower end of a fill pipe of the mold protruding from the chamber bottom.
In use of this patented apparatus as described in the patents, a supply of molten metal to be cast has been provided in the crucible by melting ingots thereof in the crucible with the crucible enclosure evacuated to a high vacuum. In casting, a mold is placed in the mold chamber inside the compartment, both chamber and compartment are sealed and evacuated, and they and the crucible enclosure are backfilled with inert gas such as argon to the same low vacuum. With the conduit valve open, the chamber is lowered through the conduit until the protruding end of the fill pipe of the mold is below the surface of the molten metal in the crucible. The chamber is then evacuated to a sufficiently high vacuum to cause the molten metal to flow through the fill passage to fill the mold cavities. After sufficient hardening of the metal in the mold cavities, the pressure is raised in the chamber and compartment, and the chamber is withdrawn from the crucible enclosure, into the compartment for removal of the mold from the chamber and compartment.
While the method and apparatus of these patents have been very successful in producing fine quality castings of air-reactive metals, the provision of the outer compartment around the mold chamber and its valved conduit connected to the crucible enclosure are expensive apparatus which complicate the operation and impose some undesirable restrictions on use. For example, the outer compartment needs sealable doors to provide access to the chamber within for inserting and removing the mold, hampering these operations. The compartment is exposed conductively to the heat of the crucible enclosure by the connecting conduit, and when the valve is open, is exposed to the heat within it, making it difficult to cool the compartment adequately. Location of the mechanism for lowering or raising the chamber in part within the compartment involves complexity of sealing the compartment about movable members extending into it, as well as prohibiting access to the mechanism and the mold while the compartment is sealed.
Other prior art, though avoiding the complex apparatus described above, fails to effectively contain the inert atmosphere within the crucible containing the molten metal.
In the inventor's related application of U.S. Pat. No. 4,791,977, the inventor attempted to prevent air contamination by maintaining the inert gas pressure in the crucible enclosure holding the melt above atmospheric pressure while the crucible enclosure was exposed to the ambient atmosphere. It was thought that the heated inert gas rising through the small opening in the crucible enclosure designed to accommodate the fill pipe would be sufficient to prevent air from passing through the opening and contacting the melt. Subsequently the inventor determined, however, that air contamination still occurred with highly reactive metals. It appeared upon investigation that the source of the problem was vertical thermal air currents or vortices circulating above the molten metal that drew air downward through rising inert gas to the crucible and into contact with the molten metal. These air currents were heretofore known in fluid mechanics as "Brillion zones".